Impact of changes in bond structure on ovonic threshold switching behaviour in GeSe2

Raman spectroscopy measurements are performed on sputtered GexSe1x thin films to identify bond presence. A large amount of homopolar bonds are found, including Ge–Ge bonds that can be attributed to Ge clustering. A time-resolved approach to Raman spectroscopy is explored to observe the effect of the high power-density laser on the sample material. Several methods are then used to tailor the structural bond homogeneity (homopolar–heteropolar bonds): annealing, varying sputter deposition pressure and the addition of dopants. In particular doping can reduce homopolar bond presence and increase heteropolar bonds presence. The impact of each dopant is supported by calculations of bond enthalpies according to Pauling equation using the approach of Lankhorst/Bicerano–Ovshinsky. Finally, in order to correlate the structural bond presence to the Ovonic Threshold Switching behaviour of (doped) GexSe1x thin films, both DC and pulsed (AC) measurement are performed on metal–insulator– metal (MIM) type test structures. It is found that minimizing homopolar bond presence is beneficial for the leakage current and electrical stability of the materia

Impact of changes in bond structure on ovonic threshold switching behaviour in GeSe2 (vol 35, pg 151, 2020)

Correction for 'Impact of changes in bond structure on ovonic threshold switching behaviour in GeSe2' by Jonas Keukelier et al., J. Mater. Chem. C, 2021, DOI: ; 10.1039/d0tc04086j

Ovonic threshold-switching GexSey chalcogenide materials : stoichiometry, trap nature, and material relaxation from first principles

Density functional theory simulations are used to identify the structural factors that define the material properties of ovonic threshold switches (OTS). They show that the nature of mobility-gap trap states in amorphous Ge-rich Ge50Se50 is related to Ge-Ge bonds, whereas in Se-rich Ge30Se70 the Ge valence-alternating-pairs and Se lone-pairs dominate. To obtain a faithful description of the electronic structure and delocalization of states, it is required to combine hybrid exchange-correlation functionals with large unit-cell models. The extent of localization of electronic states depends on the applied external electric field. Hence, OTS materials undergo structural changes during electrical cycling of the device, with a decrease in the population of less exothermic Ge-Ge bonds in favor of more exothermic Ge-Se. This reduces the amount of charge traps, which translates into coordination changes, an increase in mobility-gap, and subsequently changes in the selector-device electrical parameters. The threshold voltage drift process can be explained by natural evolution of the nonpreferred Ge-Ge bonds (or "chains"/clusters thereof) in Ge-rich GexSe1-x. The effect of extrinsic doping is shown for Si and N, which introduce strong covalent bonds into the system, increase both mobility-gap and crystallization temperature, and decrease the leakage current

Tuning of the thermal stability and ovonic threshold switching properties of GeSe with metallic and non-metallic alloying elements

In order to make 3D crossbar memory architectures viable, selector elements with highly non-linear current-voltage characteristics are required. Ovonic Threshold Switching (OTS) is a highly non-linear phenomenon observed in amorphous chalcogenides, such as GeSe, that shows promise for application in selectors. In this paper, the impact of alloying with metallic (Zr), metalloid (B, Sb), and non-metallic (C, N) elements as a function of their concentration on the thermal stability and switching properties of alloyed GeSe layers is studied. In the case of the thermal stability analysis, the key parameter that is tracked is the crystallization temperature (T-c) of the as-deposited amorphous films since OTS only occurs in amorphous materials. Using a simple metal-insulator-metal type test structure where the bottom electrode is scaled to 6 mu m, the OTS properties of the alloyed layers are also compared. The pristine leakage current (I-pris), the first fire voltage ( V-FF), and the threshold voltage ( V-th) were determined using DC and pulsed (AC) measurements. Results indicate that C alloying in combination with sufficiently high nitrogen incorporation can extend the thermal stability above 600 degrees C with only low dependence on the C content. Among the metallic and metalloid elements, crystallization temperature is strongly dependent on alloying concentration. In general, larger concentrations are needed to obtain a T-c above 400 degrees C as compared to CN alloying. Electrical characterization indicates strong dependence of the first fire voltage and the leakage current on the metallicity of the alloying element with only small to moderate concentrations required to influence electrical properties

Plasma-enhanced atomic layer deposition of nickel and cobalt phosphate for lithium ion batteries

A plasma-enhanced ALD process has been developed to deposit nickel phosphate. The process combines trimethylphosphate (TMP) plasma with oxygen plasma and nickelocene at a substrate temperature of 300 degrees C. Saturation at a growth per cycle of approximately 0.2 nm per cycle is observed for both the TMP plasma and nickelocene, while a continuous decrease in the growth per cycle is found for the oxygen plasma. From ERD, a stoichiometry of Ni-3(P0.8O3.1)(2) is measured, but by adding additional oxygen plasma after nickelocene, the composition of Ni-3(P0.9O3.7)(2) becomes even closer to stoichiometric Ni-3(PO4)(2). The as-deposited layer resulting from the process without the additional oxygen plasma is amorphous but can be crystallized into Ni2P or crystalline Ni-3(PO4)(2) by annealing under a hydrogen or helium atmosphere, respectively. The layer deposited with the additional oxygen plasma shows two X-ray diffraction peaks indicating the formation of crystalline Ni-3(PO4)(2) already during the deposition. The resulting PE-ALD deposited nickel phosphate layers were then electrochemically studied and compared to PE-ALD cobalt and iron phosphate. All phosphates need electrochemical activation at low potential first, after which reversible redox reactions are observed at a potential of approximately 2.5 V vs. Li+/Li. A relatively high capacity and good rate behavior are observed for both nickel and cobalt phosphate, which are thought to originate from either a conversion type reaction or an alloying reaction

Converting molecular layer deposited alucone films into Al2O3/alucone hybrid multilayers by plasma densification

Alucones are one of the best-known films in the Molecular Layer Deposition (MLD) field. In this work, we prove that alucone/Al2O3 nanolaminate synthesis can be successfully performed by alternating alucone MLD growth with static O2 plasma exposures. Upon plasma treatment, only the top part of the alucone is densified into Al2O3, while the rest of the film remains relatively unaltered. X-ray reflectivity (XRR) and x-ray photoelectron spectroscopy (XPS) depth profiling show that the process yields a bilayer structure, which remains stable in air. Fourier-transform infrared spectroscopy (FTIR) measurements show that Al2O3 features are generated after plasma treatment, while the original alucone features remain, confirming that plasma treatment results in a bilayer structure. Also, an intermediate carboxylate is created in the interface. Calculations of Al atom density during plasma exposure point towards a partial loss of Al atoms during plasma treatment, in addition to the removal of the glycerol backbone. The effect of different process parameters has been studied. Densification at the highest temperature possible (200ºC) has the best alucone preservation without hindering its thermal stability. In addition, operating at the lowest plasma power is found the most beneficial for the film, but there is a threshold that must be surpassed to achieve successful densification. About 70% of the original alucone film thickness can be expected to remain after densification, but thicker films may result in more diffuse interfaces. Additionally, this process has also been successfully performed in multilayers, showing real potential for encapsulation applications

Ovonic threshold-switching GexSey chalcogenide materials : stoichiometry, trap nature, and material relaxation from first principles

Density functional theory simulations are used to identify the structural factors that define the material properties of ovonic threshold switches (OTS). They show that the nature of mobility-gap trap states in amorphous Ge-rich Ge50Se50 is related to Ge-Ge bonds, whereas in Se-rich Ge30Se70 the Ge valence-alternating-pairs and Se lone-pairs dominate. To obtain a faithful description of the electronic structure and delocalization of states, it is required to combine hybrid exchange-correlation functionals with large unit-cell models. The extent of localization of electronic states depends on the applied external electric field. Hence, OTS materials undergo structural changes during electrical cycling of the device, with a decrease in the population of less exothermic Ge-Ge bonds in favor of more exothermic Ge-Se. This reduces the amount of charge traps, which translates into coordination changes, an increase in mobility-gap, and subsequently changes in the selector-device electrical parameters. The threshold voltage drift process can be explained by natural evolution of the nonpreferred Ge-Ge bonds (or "chains"/clusters thereof) in Ge-rich GexSe1-x. The effect of extrinsic doping is shown for Si and N, which introduce strong covalent bonds into the system, increase both mobility-gap and crystallization temperature, and decrease the leakage current